1. Technical Field
Embodiments of the invention relate generally to transformers used in power converters. Other embodiments relate to control circuits for such transformers.
2. Discussion of Art
The term “power converter” is most commonly applied to collections or an assembly of electrical devices that convert one form of electrical energy to another. Typically, power converters are “switching” power converters, in which multiple solid state devices are used to intermittently interrupt an input current so as to effectuate conversion of the input current to an output current having different amplitude, voltage, and/or frequency. For example, an “AC power converter” receives direct or alternating input current and produces alternating output power at design values of voltage, current, and/or frequency. By contrast, a “DC power converter” produces output power at a substantially constant output voltage and/or current.
Certain types of power converter comprise transformers. A transformer is an electromagnetic device that includes at least two “windings” or inductors, which are disposed sufficiently close together such that varying current in either of the windings can establish a varying magnetic field that excites a current in the other winding. Thus, the windings of a transformer are mutually inductive. Typically, a transformer will include a “core” of ferromagnetic material, which enhances the mutual inductance of the windings because each winding tends to magnetize the core. The mutual inductance of the windings enables power transfer from one “primary” winding to the other “secondary” winding, without direct electrical connection, via variation of the magnetic field excited in the core.
The electrical isolation provided by a transformer is a key reason why the transformer is useful in a power converter. However, it is possible for excess direct current in either winding to “saturate” the core, which means that further variation of current in that winding, above its excess value, does not effect any change in magnetization of the core. In this context, “direct current” refers to a current component of substantially constant polarity and magnitude, while “substantially” is a relative term indicating as close to a notional value as can be achieved under conventional tolerances of manufacturing and operation. When a transformer core is saturated, mutual inductance between the windings is prevented. This can result in failure of power transfer through the core. When the transformer is in use within a power converter, such failure of power transfer can have detrimental consequences for the power converter, its power supply, and its attached load. For example, when power ceases to be transferred, the load seen at the power supply drops off, which can result in an overcurrent condition at the primary side of the power converter. This condition in turn can result in a safety circuit tripping the power converter offline, or, in a worst case, physical damage to the power converter and/or its power supply.